Models--- How to use afterSave() to save others models fileds?(有中文)？

Hello! In my question I have two models Post and UserInfo: I want to after save Post and save UserInfo PostNum（+1） filed
how I can do? (我有两个模型一个Post,一个UserInfo, 我想在我保存Post之后，UserInfo的postNum也相应的加一,想问大神怎么实现？)